1. Field of the Invention
The present invention relates generally to a vibration damping device for use in an automotive engine mount or the like, for example, and more particularly, to a fluid-filled vibration damping device adapted to utilize vibration damping effect exhibited based on flow action of a non-compressible fluid filling the interior.
2. Description of the Related Art
Conventionally, vibration damping devices have been known as a one type of vibration damping connectors or vibration damping supports designed for installation between components making up a vibration transmission system and providing vibration damping linkage between the components. Such vibration damping devices have a structure in which a first mounting member and a second mounting member are elastically connected by a main rubber elastic body. Moreover, in order to obtain more enhanced vibration damping effect, there have been proposed fluid-filled vibration damping devices that utilize vibration damping effect based on flow action of a fluid, and are employed as an automotive engine mount or the like. The fluid-filled vibration damping device includes: a pressure-receiving chamber whose wall is partially constituted by the main rubber elastic body; an equilibrium chamber whose wall is partially constituted by a flexible film, the pressure-receiving chamber and the equilibrium chamber being filled with a non-compressible fluid; and an orifice passage permitting communication between the pressure-receiving chamber and the equilibrium chamber.
The fluid-filled vibration damping device exhibits excellent vibration damping effect based on flow action of a fluid or the like against vibration at specified frequencies to which the orifice passage is tuned. On the other hand, there is a problem that the device finds it difficult to exhibit effective vibration damping action against vibration in a frequency range which falls outside the tuning frequency of the orifice passage.
To address this issue, in Japanese Unexamined Patent Publication No. JP-A-10-089402 previously applied for, the applicant proposed a fluid-filled vibration damping device of pneumatic switching type which is able to exhibit effective vibration damping action through a single orifice passage against various types of vibrations having different frequencies by switching a communicating state of the orifice passage by means of a pneumatic actuator. Specifically, JP-A-10-089402 employs a structure in which the pneumatic actuator is disposed so as to face an opening of the orifice passage on the side of an equilibrium chamber with a flexible film being interposed therebetween, and the pneumatic actuator includes a center recess formed in a portion facing the opening of the orifice passage. With this arrangement, when atmospheric pressure is applied to the pneumatic actuator, the opening of the orifice passage on the side of an equilibrium chamber is covered by a movable film constituted by the flexible film, so as to set the tuning frequency of the orifice passage to a high frequency owing to the spring rigidity of the movable film. On the other hand, when a negative pressure is applied to the pneumatic actuator, the opening of the orifice passage on the side of an equilibrium chamber is opened, so that the tuning frequency of the orifice passage is set to a lower frequency in comparison with the case where the atmospheric pressure is applied.
However, additional research conducted by the inventors has led to the discovery that there is clearly room for further improvement regarding the fluid-filled vibration damping device disclosed in JP-A-10-089402. Specifically, with the structure of JP-A-10-089402, since the resonance frequency of the orifice passage is changed according to changes in the wall spring rigidity due to elasticity of the movable film, a high spring rigidity of the movable film is required in order to sufficiently change the resonance frequency of the orifice passage. However, if a high spring rigidity is set to the movable film, in a state where atmospheric pressure is applied to the pneumatic actuator, fluid flow through the orifice passage is prevented by the movable film. Thus, it may be difficult to efficiently achieve vibration damping effect on the basis of the flow action of the fluid.